(1) Field of the Invention
The present invention relates to a method of controlling revolving speeds of mill motors in a cold tandem mill. More particularly, it relates to a method of controlling motor speeds which makes it possible to obtain the desired final gauge during rolling operation, whether operation is at steady speed or it is at non-steady speed as at threading stage.
(2) Description of the Prior Art
In the manufacture of cold-rolled steel sheets, gauge accuracy is the most important control item. For the purpose of achieving such accuracy, automatic gauge control or so-called AGC technique is employed in cold-tandem mill operation. Generally, rolling operation at a tandem mill may be divided into five stages according to rolling speed, namely, threading stage for inserting the top end of the stock or a hot-rolled coil into a stand of the mill, acceleration stage for increasing the rolling speed from low at threading stage up to steady high, steady-speed operation stage where rolling is carried out with respect to a greater proportion of the coil, deceleration stage for decreasing the rolling speed, and tail-out stage, where the bottom end of the coil is dethreaded from the mill at low rolling speed. Since a major part of the coil is rolled at steady operation speed, most of the conventional AGC methods are intended for gauge control during steady-speed rolling operation, there being almost none intended for use during lower-speed rolling operation. So far, no AGC method has been proposed which can be effectively empolyed for gauge control at such stages as threading, acceleration, deceleration, and/or tail-out. Conventionally, therefore, gauge control at threading, tail-out, acceleration and deceleration stages is performed manually while operation speed is lower than the speed at which AGC system is usually actuated (several to 20 percent of steady-operation speed). This often results in no small portion of the rolled sheet being rendered off-gauge or out of tolerance limits as to gauge. Such off-gauge portion, which is naturally discarded, means decreased yield, so an effective solution to this difficulty has been strongly desired.
In order to achieve production meeting the target gauge, speed setting is made, before threading operation, with respect to roll-driving mill motors according to the draft schedule. The problem here is that the target gauge sought by mill-motor speed setting before threading is not always attainable, because some control error often occurs as the top of the coil is inserted between the rolls. Such error is due primarily to drooping characteristic control function incorporated into automatic speed control means for mill motor control. Said control means is designed to detect mill-motor speed and control it to the value according to the reference even in the event of any change being caused to the motor speed by load variation or other factor. Now, if such control function is strictly faithful to references, any erroneous setting of references may cause excessive tension to be applied to the coil at inter-stand portions thereof, with the result of coil break trouble, or conversely, it may cause no tension to be applied at all to the coil at inter-stand portions thereof, with the result of some rolling trouble. To prevent such troubles, drooping characteristic control function is usually incorporated into such control means. "Drooping characteristic control" means so called IR drop being given to automatic speed control means, which any DC motor possesses as its intrinsic characteristic. IR drop is a phenomenon that revolving speed of a motor tends to change downward (or upward) with an increase (or decrease) in a curent flowing through an armature.
Where a control function having such characteristic is incorporated in automatic speed control means, if excessive tension is going to be applied to the coil, armature current in the mill motor for the downstreamside stand will increase to slow down the motor speed (while armature current in the mill motor for the upstream-side stand will decrease to raise the motor speed) so that the tension may be moderated. Conversely, if tensionless condition develops, current in the mill-motor for downstream-side will decrease to raise the motor speed (while current in the mill motor for the upstream-side stand will increase to slow down the motor speed) so that tension may be regained. Thus, coil cut-off and rolling trouble may be prevented.
At threading stage, however, the presence of drooping characteristic is rather inconvenient. Current in mill motors is rather small at pre-threading stage at which mill-motor speed setting is made according to the predetermined conditions, but as the top end of a coil is inserted between the rolls, current tends to rapidly increase to lower the motor speed. Therefore, off-gauge is unavoidable, however appropriate the mill-motor speed setting at pre-threading stage may be. Similarly, at acceleration stage next to threading stage, or at deceleration and tail-out stages, off-gauge is likely to develop due to sudden changes in mill motor speed.